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Casting iron in a sand mold Casting may be used to create artistic sculpturesIn metalworking, casting involves pouring a liquid metal into a mold, which contains a hollow cavity of the desired shape, and then is allowed to solidify. The solidified part is also known as a casting, which is ejected or broken out of the mold to complete the process. Casting is most often used for making complex shapes that would be difficult or uneconomical to make by other methods.[1]

The casting process is subdivided into two main categories: expendable and non-expendable casting. It is further broken down by the mold material, such as sand or metal, and pouring method, such as gravity, vacuum, or low pressure.[2]

Contents [hide] 1 Terminology 2 Theory 2.1 Cooling curves 2.2 Chvorinov's rule 2.3 The gating system 2.4 Shrinkage 2.4.1 Solidification shrinkage 2.4.2 Risers and riser aids 2.4.3 Patternmaker's shrink 2.5 Mold cavity 2.6 Filling 2.7 Macrostructure 2.8 Inspection 2.8.1 Defects 3 Expendable mold casting 3.1 Waste molding of plaster 3.2 Sand casting 3.3 Plaster mold casting 3.4 Shell molding 3.5 Investment casting 3.6 Evaporative-pattern casting 3.6.1 Lost-foam casting 3.6.2 Full-mold casting 4 Non-expendable mold casting 4.1 Permanent mold casting 4.2 Die casting 4.3 Semi-solid metal casting 4.4 Centrifugal casting 4.5 Continuous casting 5 See also 6 References 6.1 Notes 6.2 Bibliography 7 External links

[edit] Terminology The casting process uses the following specialized terminology:[3]

Pattern: An approximate duplicate of the final casting used to form the mold cavity. Molding material: The material that is packed around the pattern and then the pattern is removed to leave the cavity where the casting material will be poured. Flask: The rigid wood or metal frame that holds the molding material. Cope: The top half of the pattern, flask, mold, or core. Drag: The bottom half of the pattern, flask, mold, or core. Core: An insert in the mold that produces internal features in the casting, such as holes. Core print: The region added to the pattern, core, or mold used to locate and support the core. Mold cavity: The combined open area of the molding material and core, there the metal is poured to produce the casting. Riser: An extra void in the mold that fills with molten material to compensate for shrinkage during solidification. Gating system: The network of connected channels that deliver the molten material to the mold cavities. Pouring cup or pouring basin: The part of the gating system that receives the molten material from the pouring vessel. Sprue: The pouring cup attaches to the sprue, which is the vertical part of the gating system. The other end of the sprue attaches to the runners. Runners: The horizontal portion of the gating system that connects the sprues to the gates. Gates: The controlled entrances from the runners into the mold cavities. Vents: Additional channels that provide an escape for gases generated during the pour. Parting line or parting surface: The interface between the cope and drag halves of the mold, flask, or pattern. Draft: The taper on the casting or pattern that allow it to be withdrawn from the mold Core box: The mold or die used to produce the cores. [edit] Theory Casting is a solidification process, which means the solidification phenomenon controls most of the properties of the casting. Moreover, most of the casting defects occur during solidification, such as gas porosity and solidification shrinkage.[4]

Solidification occurs in two steps: nucleation and crystal growth. In the nucleation stage solid particles form within the liquid. When these particles form their internal energy is lower than the surrounded liquid, which creates an energy interface between the two. The formation of the surface at this interface requires energy, so as nucleation occurs the material actually undercools, that is it cools below its freezing temperature, because of the extra energy required to form the interface surfaces. It then recalescences, or heats back up to its freezing temperature, for the crystal growth stage. Note that nucleation occurs on a pre-existing solid surface, because not as much energy is required for a partial interface surface, as is for a complete spherical interface surface. This can be advantageous because fine-grained castings possess better properties than coarse-grained castings. A fine grain structure can be induced by grain refinement or inoculation, which is the process of adding impurities to induce nucleation.[5]

All of the nucleations represent a crystal, which grows as the heat of fusion is extracted from the liquid until there is no liquid left. The direction, rate, and type of growth can be controlled to maximize the properties of the casting. Directional solidification is when the material solidifies at one end and proceeds to solidify to the other end; this is the most ideal type of grain growth because it allows liquid material to compensate for shrinkage.[5]

[edit] Cooling curves

Intermediate cooling rates from melt result in a dendritic microstructure. Primary and secondary dendrites can be seen in this image.See also: Cooling curves Cooling curves are important in controlling the quality of a casting. The most important part of the cooling curve is the cooling rate which affects the microstructure and properties. Generally speaking, an area of the casting which is cooled quickly will have a fine grain structure and an area which cools slowly will have a coarse grain structure. Below is an example cooling curve of a pure metal or eutectic alloy, with defining terminology.[6]

Note that before the thermal arrest the material is a liquid and after it the material is a solid; during the thermal arrest the material is converting from a liquid to a solid. Also, note that the greater the superheat the more time there is for the liquid material to flow into intricate details.[7]

The cooling rate is largely controlled by the mold material. When the liquid material is poured into the mold, the cooling begins. This happens because the heat within the molten metal flows into the relatively cooler parts of the mold. Molding materials transfer heat from the casting into the mold at different rates. For example, some molds made of plaster may transfer heat very slowly, while steel would transfer the heat quickly. Where heat should be removed quickly, the engineer will plan the mold to include special heat sinks to the mold, called chills. Fins may also be designed on a casting to extract heat, which are later removed in the cleaning (also called fettling) process. Both methods may be used at local spots in a mold where the heat will be extracted quickly. Where heat should be removed slowly, a riser or some padding may be added to a casting.[citation needed]

The above cooling curve depicts a basic situation with a pure alloy, however, most castings are of alloys, which have a cooling curve shaped as shown below.

Note that there is no longer a thermal arrest, instead there is a freezing range. The freezing range corresponds directly to the liquidus and solidus found on the phase diagram for the specific alloy.

[edit] Chvorinov's rule Main article: Chvorinov's rule The local solidification time can be calculated using Chvorinov's rule, which is:

Where t is the solidification time, V is the volume of the casting, A is the surface area of the casting that contacts the mold, n is a constant, and B is the mold constant. It is most useful in determining if a riser will solidify before the casting, because if the riser does solidify first then it is worthless.[8]

[edit] The gating system

A simple gating system for a horizontal parting mold.The gating system serves many purposes, the most important being conveying the liquid material to the mold, but also controlling shrinkage, the speed of the liquid, turbulence, and trapping dross. The gates are usually attached to the thickest part of the casting to assist in controlling shrinkage. In especially large castings multiple gates or runners may be required to introduce metal to more than one point in the mold cavity. The speed of the material is important because if the material is traveling too slow it can cool before completely filling, leading to misruns and cold shuts. If the material is moving too fast then the liquid material can erode the mold and contaminate the final casting. The shape and length of the gating system can also control how quickly the material cools; short round or square channels minimize heat loss.[9]

The gating system may be designed to minimize turbulence, depending on the material being cast. For example, steel, cast iron, and most copper alloys are turbulent insensitive, but aluminium and magnesium alloys are turbulent sensitive. The turbulent insensitive materials usually have a short and open gating system to fill the mold as quickly as possible. However, for turbulent sensitive materials short sprues are used to minimize the distance the material must fall when entering the mold. Rectangular pouring cups and tapered sprues are used to prevent the formation of a vortex as the material flows into the mold; these vortices tend to suck gas and oxides into the mold. A large sprue well is used to dissipate the kinetic energy of the liquid material as it falls down the sprue, decreasing turbulence. The choke, which is the smallest cross-sectional area in the gating system used to control flow, can be placed near the sprue well to slow down and smooth out the flow. Note that on some molds the choke is still placed on the gates to make separation of the part easier, but induces extreme turbulence.[10] The gates are usually attached to the bottom of the casting to minimize turbulence and splashing.[9]

The gating system may also be designed to trap dross. One method is to take advantage of the fact that some dross has a lower density than the base material so it floats to the top of the gating system. Therefore long flat runners with gates that exit from the bottom of the runners can trap dross in the runners; note that long flat runners will cool the material more rapidly than round or square runners. For materials where the dross is a similar density to the base material, such as aluminium, runner extensions and runner wells can be advantageous. These take advantage of the fact that the dross is usually located at the beginning of the pour, therefore the runner is extended past the last gate(s) and the contaminates are contained in the wells. Screens or filters may also be used to trap contaminates.[10]

It is important to keep the size of the gating system small, because it all must be cut from the casting and remelted to be reused. The efficiency, or yield, of a casting system can be calculated by dividing the weight of the casting by the weight of the metal poured. Therefore, the higher the number the more efficient the gating system/risers.[11]

[edit] Shrinkage There are three types of shrinkage: shrinkage of the liquid, solidification shrinkage and patternmaker's shrinkage. The shrinkage of the liquid is rarely a problem because more materials flowing into the mold behind it. Solidification shrinkage occurs because metals are less dense as a liquid than a solid, so during solidification the metal density dramatically increases. Patternmaker's shrinkage refers to the shrinkage that occurs when the material is cooled from the solidification temperature to room temperature, which occurs due to thermal contraction.[12]

[edit] Solidification shrinkage Solidification shrinkage of various metals[13][14] Metal Percentage Aluminium 6.6 Copper 4.9 Magnesium 4.0 or 4.2 Zinc 3.7 or 6.5 Low carbon steel 2.5–3.0 High carbon steel 4.0 White cast iron 4.0–5.5 Gray cast iron −2.5–1.6 Ductile cast iron −4.5–2.7 Most materials shrink as they solidify, but, as the table to the right shows, a few materials do not, such as gray cast iron. For the materials that do shrink upon solidification the type of shrinkage depends on how wide the freezing range is for the material. For materials with a narrow freezing range, less than 50 °C (122 °F),[15] a pipe type cavity forms in the center of the cavity, because the outer shell freezes first and progressively solidifies to the center. Pure and eutectic metals usually have narrow solidification ranges. These materials tend to form a skin in open air molds, therefore they are known as skin forming alloys.[15] For materials with a wide freezing range, greater than 110 °C (230 °F),[15] much more of the casting occupies the mushy or slushy zone (the temperature range between the solidus and the liquidus), which leads to small pockets of liquid trapped throughout and ultimately porosity. These castings tend to have poor ductility, toughness, and fatigue resistance. Moreover, for these types of materials to be fluid-tight a secondary operation is required to impregnate the casting with a lower melting point metal or resin.[13][16]

For the materials that have narrow solidification ranges pipes can be overcome by designing the casting to promote directional solidification, which means the casting freezes first at the point farthest from the gate, then progressively solidifies towards the gate. This allows a continuous feed of liquid material to be present at the point of solidification to compensate for the shrinkage. Note that there is still a shrinkage void where the final material solidifies, but if designed properly this will be in the gating system or riser.[13]

[edit] Risers and riser aids

Different types of risersMain articles: Riser (casting) and chill (casting) Risers, also known as feeders, are the most common way of providing directional solidification. It supplies liquid metal to the solidifying casting to compensate for solidification shrinkage. For a riser to work properly the riser must solidify after the casting, otherwise it cannot supply liquid metal to shrinkage within the casting. Risers add cost to the casting because it lowers the yield of each casting; i.e. more metal is lost as scrap for each casting. Another way to promote directional solidification is by adding chills to the mold. A chill is any material which will conduct heat away from the casting more rapidly that the material used for molding.[17]

Risers are classified by three criteria. The first is if the riser is open to the atmosphere, if it is then its called an open riser, otherwise its known as a blind type. The second criterion is where the riser is located; if it is located on the casting then it is known as a top riser and if it is located next to the casting it is known as a side riser. Finally, if riser is located on the gating system so that it fills after the molding cavity, it is known as a live riser or hot riser, but if the riser fills with materials that's already flowed through the molding cavity it is known as a dead riser or cold riser.[11]

Riser aids are items used to assist risers in creating directional solidification or reducing the number of risers required. One of these items are chills which accelerate cooling in a certain part of the mold. There are two types: external and internal chills. External chills are masses of high-heat-capacity and high-thermal-conductivity material that are placed on an edge of the molding cavity. Internal chills are pieces of the same metal that is being poured, which are placed inside the mold cavity and become part of the casting. Insulating sleeves and toppings may also be installed around the riser cavity to slow the solidification of the riser. Heater coils may also be installed around or above the riser cavity to slow solidification.[18]

[edit] Patternmaker's shrink Typical patternmaker's shrinkage of various metals[19] Metal Percentage in/ft Aluminium 1.0–1.3 1⁄8–5⁄32 Brass 1.5 3⁄16 Magnesium 1.0–1.3 1⁄8–5⁄32 Cast iron 0.8–1.0 1⁄10–1⁄8 Steel 2.5–3.0 3⁄16–1⁄4 Shrinkage after solidification can be dealt with by using an oversized pattern designed specifically for the alloy used. Contraction rules, or shrink rules, are used to make the patterns oversized to compensate for this type of shrinkage.[19] These rulers are up to 2% oversize, depending on the material being cast.[18] These rulers are mainly referred to by their percentage change. A pattern made to match an existing part would be made as follows: First, the existing part would be measured using a standard ruler, then when constructing the pattern, the pattern maker would use a contraction rule, ensuring that the casting would contract to the correct size.

Note that patternmaker's shrinkage does not take phase change transformations into account. For example, eutectic reactions, martensitic reactions, and graphitization can cause expansions or contractions.[19]

[edit] Mold cavity The mold cavity of a casting does not reflect the exact dimensions of the finished part due to a number of reasons. These modifications to the mold cavity are known as allowances and account for patternmaker's shrinkage, draft, machining, and distortion. In non-expendable processes, these allowances are imparted directly into the permanent mold, but in expendable mold processes they are imparted into the patterns, which later form the mold cavity.[19] Note that for non-expendable molds an allowance is required for the dimensional change of the mold due to heating to operating temperatures.[20]

For surfaces of the casting that are perpendicular to the parting line of the mold a draft must be included. This is so that the casting can be release in non-expendable processes or the pattern can be released from the mold without destroying the mold in expendable processes. The required draft angle depends on the size and shape of the feature, the depth of the mold cavity, how the part or pattern is being removed from the mold, the pattern or part material, the mold material, and the process type. Usually the draft is not less than 1%.[19]

The machining allowance varies drastically from one process to another. Sand castings generally have a rough surface finish, therefore need a greater machining allowance, whereas die casting has a very fine surface finish, which may not need any machining tolerance. Also, the draft may provide enough of a machining allowance to begin with.[20]

The distortion allowance is only necessary for certain geometries. For instance, U-shaped castings will tend to distort with the legs splaying outward, because the base of the shape can contract while the legs are constrained by the mold. This can be overcome by designing the mold cavity to slope the leg inward to begin with. Also, long horizontal sections tend to sag in the middle if ribs are not incorporated, so a distortion allowance may be required.[20]

Cores may be used in expendable mold processes to produce internal features. The core can be of metal but it is usually done in sand.

[edit] Filling

This section requires expansion. 

There are a few common methods for filling the mold cavity: gravity, low-pressure, high-pressure, and a vacuum.

The first patented vacuum casting machine and process dates to 1879.[21]

[edit] Macrostructure The grain macrostructure in ingots and most castings have three distinct regions or zones: the chill zone, columnar zone, and equiaxed zone. The image below depicts these zones.

The chill zone is named so because it occurs at the walls of the mold where the wall chills the material. Here is where the nucleation phase of the solidification process takes place. As more heat is removed the grains grow towards the center of the casting. These are thin, long columns that are perpendicular to the casting surface, which are undesirable because they have anisotropic properties. Finally, in the center the equiaxed zone contains spherical, randomly oriented crystals. These are desirable because they have isotropic properties. The creation of this zone can be promoted by using a low pouring temperature, alloy inclusions, or inoculants.[8]

[edit] Inspection Common inspection methods for steel castings are magnetic particle and liquid penetrant.[22] Common inspection methods for aluminum castings are radiography, ultrasonic, and liquid penetrant.[23]

[edit] Defects Main article: Casting defects There are a number of problems that can be encountered during the casting process. The main types are: gas porosity, shrinkage defects, mold material defects, pouring metal defects, and metallurgical defects.

[edit] Expendable mold casting Expendable mold casting is a generic classification that includes sand, plastic, shell, plaster, and investment (lost-wax technique) moldings. This method of mold casting involves the use of temporary, non-reusable molds.

[edit] Waste molding of plaster

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A durable plaster intermediate is often used as a stage toward the production of a bronze sculpture or as a pointing guide for the creation of a carved stone. With the completion of a plaster, the work is more durable (if stored indoors) than a clay original which must be kept moist to avoid cracking. With the low cost plaster at hand, the expensive work of bronze casting or stone carving may be deferred until a patron is found, and as such work is considered to be a technical, rather than artistic process, it may even be deferred beyond the lifetime of the artist.

In waste molding a simple and thin plaster mold, reinforced by sisal or burlap, is cast over the original clay mixture. When cured, it is then removed from the damp clay, incidentally destroying the fine details in undercuts present in the clay, but which are now captured in the mold. The mold may then at any later time (but only once) be used to cast a plaster positive image, identical to the original clay. The surface of this plaster may be further refined and may be painted and waxed to resemble a finished bronze casting.

[edit] Sand casting Main article: Sand casting Sand casting is one of the most popular and simplest types of casting that has been used for centuries. Sand casting allows for smaller batches to be made compared to permanent mold casting and at a very reasonable cost. Not only does this method allow manufacturers to create products at a low cost, but there are other benefits to sand casting, such as very small size operations. From castings that fit in the palm of your hand to train beds (one casting can create the entire bed for one rail car), it can all be done with sand casting. Sand casting also allows most metals to be cast depending on the type of sand used for the molds.[24]

Sand casting requires a lead time of days for production at high output rates (1–20 pieces/hr-mold) and is unsurpassed for large-part production. Green (moist) sand has almost no part weight limit, whereas dry sand has a practical part mass limit of 2,300–2,700 kg (5,100–6,000 lb). Minimum part weight ranges from 0.075–0.1 kg (0.17–0.22 lb). The sand is bonded together using clays, chemical binders, or polymerized oils (such as motor oil). Sand can be recycled many times in most operations and requires little maintenance.

[edit] Plaster mold casting Main article: Plaster mold casting Plaster casting is similar to sand casting except that plaster of paris is substituted for sand as a mold material. Generally, the form takes less than a week to prepare, after which a production rate of 1–10 units/hr-mold is achieved, with items as massive as 45 kg (99 lb) and as small as 30 g (1 oz) with very good surface finish and close tolerances.[25] Plaster casting is an inexpensive alternative to other molding processes for complex parts due to the low cost of the plaster and its ability to produce near net shape castings. The biggest disadvantage is that it can only be used with low melting point non-ferrous materials, such as aluminium, copper, magnesium, and zinc.[26]

[edit] Shell molding Main article: Shell molding Shell molding is similar to sand casting, but the molding cavity is formed by a hardened "shell" of sand instead of flask filled with sand. The sand is finer than sand casting sand and is mixed with a resin so that it can be heated by the pattern and harden into a shell around the pattern. Because of the resin it gives a much finer surface finish. The process is easily automated and more precise than sand casting. Common metals that are cast include cast iron, aluminium, magnesium, and copper alloys. This process is ideal for complex items that are small to medium sized.

[edit] Investment casting

An investment-cast valve coverMain article: Investment casting See also: Lost-wax casting Investment casting (known as lost-wax casting in art) is a process that has been practised for thousands of years, with the lost-wax process being one of the oldest known metal forming techniques. From 5000 years ago, when beeswax formed the pattern, to today’s high technology waxes, refractory materials and specialist alloys, the castings ensure high-quality components are produced with the key benefits of accuracy, repeatability, versatility and integrity.

Investment casting derives its name from the fact that the pattern is invested, or surrounded, with a refractory material. The wax patterns require extreme care for they are not strong enough to withstand forces encountered during the mold making. One advantage of investment casting is that the wax can be reused.[25]

The process is suitable for repeatable production of net shape components from a variety of different metals and high performance alloys. Although generally used for small castings, this process has been used to produce complete aircraft door frames, with steel castings of up to 300 kg and aluminium castings of up to 30 kg. Compared to other casting processes such as die casting or sand casting, it can be an expensive process, however the components that can be produced using investment casting can incorporate intricate contours, and in most cases the components are cast near net shape, so requiring little or no rework once cast.

[edit] Evaporative-pattern casting Main article: Evaporative-pattern casting This is a class of casting processes that use pattern materials that evaporate during the pour, which means there is no need to remove the pattern material from the mold before casting. The two main processes are lost-foam casting and full-mold casting.

[edit] Lost-foam casting Main article: Lost-foam casting Lost-foam casting is a type of evaporative-pattern casting process that is similar to investment casting except foam is used for the pattern instead of wax. This process takes advantage of the low boiling point of foam to simplify the investment casting process by removing the need to melt the wax out of the mold.

[edit] Full-mold casting Main article: Full-mold casting Full-mold casting is an evaporative-pattern casting process which is a combination of sand casting and lost-foam casting. It uses a expanded polystyrene foam pattern which is then surrounded by sand, much like sand casting. The metal is then poured directly into the mold, which vaporizes the foam upon contact.

[edit] Non-expendable mold casting

The permanent molding processNon-expendable mold casting differs from expendable processes in that the mold need not be reformed after each production cycle. This technique includes at least four different methods: permanent, die, centrifugal, and continuous casting.

[edit] Permanent mold casting Main articles: Permanent mold casting, low-pressure permanent mold casting, and vacuum permanent mold casting Permanent mold casting is metal casting process that employs reusable molds ("permanent molds"), usually made from metal. The most common process uses gravity to fill the mold, however gas pressure or a vacuum are also used. A variation on the typical gravity casting process, called slush casting, produces hollow castings. Common casting metals are aluminum, magnesium, and copper alloys. Other materials include tin, zinc, and lead alloys and iron and steel are also cast in graphite molds. Permanent molds, while lasting more than one casting still have a limited life before wearing out.

[edit] Die casting Main article: Die casting The die casting process forces molten metal under high pressure into mold cavities (which are machined into dies). Most die castings are made from nonferrous metals, specifically zinc, copper, and aluminium based alloys, but ferrous metal die castings are possible. The die casting method is especially suited for applications where many small to medium sized parts are needed with good detail, a fine surface quality and dimensional consistency.

[edit] Semi-solid metal casting Main article: Semi-solid metal casting Semi-solid metal (SSM) casting is a modified die casting process that reduces or eliminates the residual porosity present in most die castings. Rather than using liquid metal as the feed material, SSM casting uses a higher viscosity feed material that is partially solid and partially liquid. A modified die casting machine is used to inject the semi-solid slurry into re-usable hardened steel dies. The high viscosity of the semi-solid metal, along with the use of controlled die filling conditions, ensures that the semi-solid metal fills the die in a non-turbulent manner so that harmful porosity can be essentially eliminated.

Used commercially mainly for aluminium and magnesium alloys, SSM castings can be heat treated to the T4, T5 or T6 tempers. The combination of heat treatment, fast cooling rates (from using un-coated steel dies) and minimal porosity provides excellent combinations of strength and ductility. Other advantages of SSM casting include the ability to produce complex shaped parts net shape, pressure tightness, tight dimensional tolerances and the ability to cast thin walls.[27].

[edit] Centrifugal casting Main article: Centrifugal casting Centrifugal casting is both gravity- and pressure-independent since it creates its own force feed using a temporary sand mold held in a spinning chamber at up to 900 N. Lead time varies with the application. Semi- and true-centrifugal processing permit 30-50 pieces/hr-mold to be produced, with a practical limit for batch processing of approximately 9000 kg total mass with a typical per-item limit of 2.3-4.5 kg.

Industrially, the centrifugal casting of railway wheels was an early application of the method developed by German industrial company Krupp and this capability enabled the rapid growth of the enterprise.

Small art pieces such as jewelry are often cast by this method using the lost wax process, as the forces enable the rather viscous liquid metals to flow through very small passages and into fine details such as leaves and petals. This effect is similar to the benefits from vacuum casting, also applied to jewelry casting.

[edit] Continuous casting Main article: Continuous casting Continuous casting is a refinement of the casting process for the continuous, high-volume production of metal sections with a constant cross-section. Molten metal is poured into an open-ended, water-cooled copper mold, which allows a 'skin' of solid metal to form over the still-liquid centre. The strand, as it is now called, is withdrawn from the mold and passed into a chamber of rollers and water sprays; the rollers support the thin skin of the strand while the sprays remove heat from the strand, gradually solidifying the strand from the outside in. After solidification, predetermined lengths of the strand are cut off by either mechanical shears or travelling oxyacetylene torches and transferred to further forming processes, or to a stockpile. Cast sizes can range from strip (a few millimetres thick by about five metres wide) to billets (90 to 160 mm square) to slabs (1.25 m wide by 230 mm thick). Sometimes, the strand may undergo an initial hot rolling process before being cut.

Continuous casting is used due to the lower costs associated with continuous production of a standard product, and also increases the quality of the final product. Metals such as steel, copper and aluminium are continuously cast, with steel being the metal with the greatest tonnages cast using this method.

[edit] Kohloff, Frederick H.; Sylvia, J. Gerin; American Foundry Society (2003), Technology of Metalcasting, American Foundry Society, ISBN 9780874332575 .

Heat treatment

Heat treatment is a method used to alter the physical, and sometimes chemical, properties of a material. The most common application is metallurgical. Heat treatments are also used in the manufacture of many other materials, such as glass. Heat treatment involves the use of heating or chilling, normally to extreme temperatures, to achieve a desired result such as hardening or softening of a material. Heat treatment techniques include annealing, case hardening, precipitation strengthening, tempering and quenching. It is noteworthy that while the term heat treatment applies only to processes where the heating and cooling are done for the specific purpose of altering properties intentionally, heating and cooling often occur incidentally during other manufacturing processes such as hot forming or welding.

Contents [hide] 1 Processes 1.1 Annealing 1.2 Hardening and tempering (quenching and tempering) 1.3 Precipitation hardening 1.4 Selective hardening 1.5 Case hardening 2 Specification 2.1 Case hardening 2.2 Through hardening 2.3 Annealing 3 See also 4 References 5 Further reading 6 External links

[edit] Processes

Heat treating furnace at 1,800 °F (980 °C)Metallic materials consist of a microstructure of small crystals called "grains" or crystallites. The nature of the grains (i.e. grain size and composition) is one of the most effective factors that can determine the overall mechanical behavior of the metal. Heat treatment provides an efficient way to manipulate the properties of the metal by controlling rate of diffusion, and the rate of cooling within the microstructure.

Complex heat treating schedules are often devised by metallurgists to optimize an alloy's mechanical properties. In the aerospace industry, a superalloy may undergo five or more different heat treating operations to develop the desired properties. This can lead to quality problems depending on the accuracy of the furnace's temperature controls and timer.

[edit] Annealing Main article: Annealing (metallurgy) Annealing is a technique used to recover cold work and relax stresses within a metal. Annealing typically results in a soft, ductile metal. When an annealed part is allowed to cool in the furnace, it is called a "full anneal" heat treatment. When an annealed part is removed from the furnace and allowed to cool in air, it is called a "normalizing" heat treatment. During annealing, small grains recrystallize to form larger grains. In precipitation hardening alloys, precipitates dissolve into the matrix, "solutionizing" the alloy.

Typical annealing processes include, "normalizing", "stress relief" annealing to recover cold work, and full annealing.

[edit] Hardening and tempering (quenching and tempering) Main article: Quench To harden by quenching, a metal (usually steel or cast iron) must be heated into the austenitic crystal phase and then quickly cooled. Depending on the alloy and other considerations (such as concern for maximum hardness vs. cracking and distortion), cooling may be done with forced air or other gas (such as nitrogen), oil, polymer dissolved in water, or brine. Upon being rapidly cooled, a portion of austenite (dependent on alloy composition) will transform to martensite, a hard brittle crystalline structure. The quenched hardness of a metal depends upon its chemical composition and quenching method. Cooling speeds, from fastest to slowest, go from polymer (i.e.silicon), brine, fresh water, oil, and forced air. However, quenching a certain steel too fast can result in cracking, which is why High-tensile steels like AISI 4140 should be quenched in oil, tool steels such as 2767 or H13 hot work tool steel should be quenched in forced air, and low alloy or medium-tensile steels such as XK1320 or AISI 1040 should be quenched in brine or water. However, metals such as austenitic stainless steel (304, 316), and copper, produce an opposite effect when these are quenched; they anneal. Austenitic stainless steels must be quench-annealed to become fully corrosion resistant, as they work-harden significantly.

Untempered martensite, while very hard and strong, is too brittle to be useful for most applications. A method for alleviating this problem is called tempering. Most applications require that quenched parts be tempered (heat treated at a low temperature, often three hundred degrees Fahrenheit or one hundred fifty degrees Celsius) to impart some toughness. Higher tempering temperatures (may be up to thirteen hundred degrees Fahrenheit or seven hundred degrees Celsius, depending on alloy and application) are sometimes used to impart further ductility, although some yield strength is lost.

[edit] Precipitation hardening Main article: Precipitation hardening Some metals are classified as precipitation hardening metals. When a precipitation hardening alloy is quenched, its alloying elements will be trapped in solution, resulting in a soft metal. Aging a "solutionized" metal will allow the alloying elements to diffuse through the microstructure and form intermetallic particles. These intermetallic particles will nucleate and fall out of solution and act as a reinforcing phase, thereby increasing the strength of the alloy. Alloys may age "naturally" meaning that the precipitates form at room temperature, or they may age "artificially" when precipitates only form at elevated temperatures. In some applications, naturally aging alloys may be stored in a freezer to prevent hardening until after further operations - assembly of rivets, for example, may be easier with a softer part.

Examples of precipitation hardening alloys include 2000 series, 6000 series, and 7000 series aluminium alloy, as well as some superalloys and some stainless steels.

[edit] Selective hardening Some techniques allow different areas of a single object to receive different heat treatments. This is called differential hardening. It is common in high quality knives and swords. The Chinese jian is one of the earliest known examples of this, and the Japanese katana the most widely known. The Nepalese Khukuri is another example.

[edit] Case hardening Main article: Case hardening Case hardening is a process in which an alloying element, most commonly carbon or nitrogen, diffuses into the surface of a monolithic metal. The resulting interstitial solid solution is harder than the base material, which improves wear resistance without sacrificing toughness.

Laser surface engineering is a surface treatment with high versatility, selectivity and novel properties. Since the cooling rate is very high in laser treatment, metastable even metallic glass can be obtained by this method.

[edit] Specification Usually the end condition is specified instead of the process used in heat treatment.[1]

[edit] Case hardening Case hardening is specified by hardness and case depth. The case depth can be specified in two ways: total case depth or effective case depth. The total case depth is the true depth of the case. The effective case depth is the depth of the case that has a hardness equivalent of HRC50; this is checked on a Tukon microhardness tester. This value can be roughly approximated as 65% of the total case depth; however the chemical composition and hardenability can affect this approximation. If neither type of case depth is specified the total case depth is assumed.[1]

For case hardened parts the specification should have a tolerance of at least ±0.005 in (0.13 mm). If the part is to be ground after heat treatment, the case depth is assumed to be after grinding.[1]

The Rockwell hardness scale used for the specification depends on the depth of the total case depth, as shown in the table below. Usually hardness is measured on the Rockwell "C" scale, but the load used on the scale will penetrate through the case if the case is less than 0.030 in (0.76 mm). Using Rockwell "C" for a thinner case will result in a false reading.[1]

Rockwell scale required for various case depths[1] Total case depth, min. [in] Rockwell scale 0.030 C 0.024 A 0.021 45N 0.018 30N 0.015 15N Less than 0.015 "File hard"

For cases that are less than 0.015 in (0.38 mm) thick a Rockwell scale cannot reliably be used, so "file hard" is specified instead.[1]

When specifying the hardness either a range should be given or the minimum hardness specified. If a range is specified at least 5 points should be given.[1]

[edit] Through hardening Only hardness is listed for through hardening. It is usually in the form of HRC with at least a five point range.[1]

[edit] Annealing The hardness for an annealing process is usually listed on the HRB scale as a maximum value.[1]

Abrasive blasting From Wikipedia, the free encyclopedia

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Sandblasting a stone wall Diesel powered compressor used as an air supply for sandblasting This article has multiple issues. Please help improve the article or discuss these issues on the talk page. It needs additional references or sources for verification. Tagged since February 2010. It may need to be wikified to meet Wikipedia's quality standards. Tagged since September 2009. Its lead section requires expansion. Tagged since September 2009.

Abrasive blasting is the operation of forcibly propelling a stream of abrasive material against a surface under high pressure to smooth a rough surface, roughen a smooth surface, shape a surface, or remove surface contaminants. The first abrasive blasting process was patented by Benjamin Chew Tilghman on October 18, 1870.[citation needed]

There are several variants of the process, such as bead blasting, sandblasting, shot blasting and sodablasting.

Contents [hide] 1 Operations 1.1 Bead blasting 1.2 Wheel blasting 1.3 Hydro-blasting 1.4 Micro-abrasive blasting 1.5 Automated blasting 2 Equipment 2.1 Portable blast equipment 2.2 Blast cabinet 2.3 Blast room 3 Media 4 Safety 5 Applications 6 See also 7 References 8 Bibliography

[edit] Operations Abrasive blasting is a method of propelling abrasive using a compressed gas (typically air) or pressurized liquid (typically water). Common forms of abrasive blasting include bead blasting, sandblasting, shot blasting and sodablasting.

[edit] Bead blasting Bead blasting is the process of removing surface deposits by applying fine glass beads at a low pressure without damaging the surface.

It is used to clean calcium deposits from pool tiles or any other surfaces, and removes embedded fungus and brightens grout color. This process is notably used as an efficiently popular way to clean tile surfaces in swimming pools. It is also used in autobody work to remove paint.

[edit] Wheel blasting In wheel blasting, a wheel uses centrifugal force to propel the abrasive against the substrate. It is typically categorized as an airless blasting operation because there is no propellant (gas or liquid) used. A wheel machines is a high-power, high-efficiency blasting operation with recyclable abrasive (typically steel or stainless steel shot, cut wire, grit or similar sized pellets). Specialized wheel blast machines propel plastic abrasive in a cryogenic chamber, and is usually used for deflashing plastic and rubber components. The size of the wheel blast machine, and the number and power of the wheels vary considerably depending on the parts to be blasted as well as on the expected result and efficiency.

[edit] Hydro-blasting Hydro-blasting, commonly known as water blasting, is popular because it usually requires only one operator. In hydro-blasting, a highly pressured stream of water is used to remove old paint, chemicals, or buildup without damaging the original surface. This method is ideal for cleaning internal and external surfaces because the operator is generally able to send the stream of water into places that are difficult to reach using other methods. Another benefit of hydro-blasting is the ability to recapture and reuse the water, reducing waste and the impact on the environment.

[edit] Micro-abrasive blasting Micro-abrasive blasting is dry abrasive blasting process that uses small nozzles (typically 0.25 mm to 1.5 mm diameter) to deliver a fine stream of abrasive accurately to either a small part (mm size) or a small area on a larger part. Generally the area to be blasted is from about 1 mm to only a few cm at most. Also known as pencil blasting, the fine jet of abrasive is accurate enough to write directly on glass and delicate enough to cut a pattern in an eggshell. The abrasive media particle sizes range from 10 micrometres up to about 150 micrometres. Higher pressures are often required. The abrasive media is generally not recycled, since the particles tend to shatter on impact or lose their sharp edges.

The most common micro-abrasive blasting systems are commercial bench-mounted units consisting of a power supply and mixer, exhaust hood, nozzle and gas supply. The nozzle can be hand-held or fixture mounted for automatic operation. Either the nozzle or part can be moved in automatic operation.

[edit] Automated blasting

This section requires expansion. 

A fully automated blasting system usually includes contained surface preparation and coating applications.

[edit] Equipment

Device used for adding sand to the compressed air (top of which is a sieve for adding the sand)[edit] Portable blast equipment Dry abrasive blasting applications are typically powered by a diesel air compressor. A pressurized vessel contains the abrasive media and meters it into the compressed air stream. Portable dry blasting may not contain or minimize the dust generated from the operation.

In wet blasting, the abrasive is introduced into a pressurized stream of water or other liquid, creating a slurry. Wet blasting is often used in applications where the minimal dust generation is desired. Portable applications may or may not recycle the abrasive.

[edit] Blast cabinet

A sand blasting cabinetA blast cabinet is essentially a closed loop system that allows the operator to blast the part and recycle the abrasive. It usually consists of four components; the containment (cabinet), the abrasive blasting system, the abrasive recycling system and the dust collection. The operator blasts the parts from the outside of the cabinet by placing his arms in gloves attached to glove holes on the cabinet, viewing the part through a view window, turning the blast on and off using a foot pedal or treadle. Automated blast cabinets are also used to process large quantities of the same component and may incorporate multiple blast nozzles and a part conveyance system.

There are three systems typically used in a blast cabinet. Two, siphon and pressure, are dry and one is wet:

1. A siphon blast system (aka suction blast system) uses the compressed air to create vacuum in a chamber (known as the blast gun). The negative pressure pulls abrasive into the blast gun where the compressed air directs the abrasive through a blast nozzle. The abrasive mixture travels through a nozzle that's directs the particles toward the surface or workpiece.

Nozzles come in a variety of shapes, sizes, and materials. Tungsten carbide is the liner material most often used for mineral abrasives. Silicon carbide and boron carbide nozzles are more wear resistant and are often used with harder abrasives such as aluminum oxide. Inexpensive abrasive blasting systems and smaller cabinets use ceramic nozzles.

2. In a pressure blast system, the abrasive is stored in the pressure vessel then sealed. The vessel is pressurized to the same pressure as the blast hose attached to the bottom of the pressure vessel. The abrasive is metered into the blast hose and conveyed by the compressed gas through the blast nozzle.

3. Wet blast cabinets use a system that injects the abrasive/liquid slurry into a compressed gas stream. Wet blasting is typically used when the heat produced by friction in dry blasting would damage the part.

[edit] Blast room A blast room is a larger version of a blast cabinet and the blast operator works inside the room. A blast room includes three of the four components of a blast cabinet: the containment structure, the abrasive blasting system and the dust collector. Most blast rooms have recycling systems ranging from manual sweeping and shoveling the abrasive back into the blast pot to full reclaim floors that convey the abrasive pneumatically or mechanically to a device that cleans the abrasive prior to recycling.

[edit] Media Silica sand: Silica sand is the most commonly used abrasive. It tends to break up quickly, creating large quantities of dust, exposing the operator to the potential development of silicosis, a debilitating lung disease. To counter this hazard, silica sand for blasting is often coated with resins to control the dust.

In the early 1900s, it was assumed that sharp-edged grains provided the best performance, but this was later demonstrated to not be correct.[1]

Metallic, synthetic and mineral: These other types of abrasives are growing in popularity due to the low dust creation and ease of reclamation of material.

Organic: Typically, crushed nut shells or fruit kernels. These soft abrasives are used to avoid damaging the underlying material such when cleaning brick or stone, removing graffiti, or the removal of coatings from printed circuit boards being repaired. Sodablasting uses baking soda (sodium bicarbonate) which is extremely friable, the micro fragmentation on impact exploding away surface materials without damage to the substrate.

Other materials for sandblasting include carborundum grit,steel shot and grit, copper slag, powdered slag, glass beads (bead blasting), metal pellets, dry ice, garnet[2], powdered abrasives of various grades. Multi-media blasters are specially designed to handle multiple blast abrasives.

Many coarser media used in sandblasting often result in energy being given off as sparks or light on impact. The colours and size of the spark or glow varies significantly, with heavy bright orange sparks from steel shot blasting, to a faint blue glow (often invisible in daylight) from garnet grit.

[edit] Safety Cleaning operations using abrasive blasting can present risks for workers' health and safety, especially in air blasting applications. Although many abrasives used in blasting booths are not hazardous in themselves, (steel shot and grit, cast iron, aluminum oxide [aka corundum], garnet, plastic abrasive and glass bead), other abrasives (silica sand, copper slag, nickel slag, and staurolite) have varying degrees of hazard (typically free silica or heavy metals). However, in all cases their use can present serious danger to operators, such as burns due to projections (with skin or eye lesions), falls due to walking on round shots scattered on the ground, exposure to hazardous dusts, heat exhaustion, creation of an explosive atmosphere, and exposure to excessive noise. Blasting booths and portable blaster's equipment have been adapted to these dangers.

OSHA (Occupational Safety and Health Administration) mandates engineered solutions to potential hazards, however lsilica sand continue to be allowed even though most commonly used blast helmets are not sufficiently effective at protecting the blast operator. (Respiratory protection is approved by NIOSH - National Institute for Occupational Safety and Health).

Typical safety equipment for operators include:

Positive pressure blast hood or helmet - The hood or helmet includes a head suspension system to allow the device to move with the operator's head, a view window with replaceable lens or lens protection and an air feed hose. Grade D air supply - The air feed hose is typically attached to a grade D pressurized air supply. Grade D air is mandated by OSHA to protect the worker from hazardous gases. It includes a pressure regulator, air filtration and a carbon monoxide alarm. Ear protection - ear muffs or ear plugs. Body protection - Body protection varies by application but usually consists of gloves and overalls or a leather coat and chaps. Professionals would wear a cordura/canvas blast suit. In the past, when sandblasting was performed as an open-air job, the worker was exposed to risk of injury from the flying material and lung damage from inhaling the dust. The silica dust produced in the sandblasting process would cause silicosis after sustained inhalation of the dust. In 1918, the first sandblasting enclosure was built, which protected the worker with a viewing screen, revolved around the workpiece, and used an exhaust fan to draw dust away from the worker's face.[3]

Several countries and territories now regulate sandblasting such that it may only be performed in a controlled environment using ventilation, protective clothing and breathing air supply.

[edit] Applications Specialized applications

The lettering and engraving on most modern cemetery monuments and markers is created by abrasive blasting.

Sandblasting can also be used to produce three dimensional signage. This type of signage is considered to be a higher end product as compared to the flat signs. These signs often incorporate gold leaf overlay and sometimes crushed glass backgrounds which is called smalts.

Sandblasting can be used to refurbish buildings or create works of art (carved or frosted glass). Modern masks and resists facilitate this process, producing accurate results.

Sandblasting techniques are used for cleaning boat hulls, as well as brick, stone and concrete work. Sandblasting is used for cleaning industrial as well as commercial structures, but is rarely used for non-metallic workpieces.

Machining From Wikipedia, the free encyclopedia Jump to: navigation, search

New Guinea in 1943. Mobile Machine Shop truck of the US Army with machinists working on automotive parts."Machine shop" redirects here. For the record label, see Machine Shop Recordings.

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Conventional machining, one of the most important material removal methods, is a collection of material-working processes in which power-driven machine tools, such as lathes, milling machines, and drill presses, are used with a sharp cutting tool to mechanically cut the material to achieve the desired geometry. Machining is a part of the manufacture of almost all metal products, and it is common for other materials, such as wood and plastic, to be machined. A person who specializes in machining is called a machinist. A room, building, or company where machining is done is called a machine shop. Much of modern day machining is controlled by computers using computer numerical control (CNC) machining. Machining can be a business, a hobby, or both.

The precise meaning of the term "machining" has evolved over the past 1.5 centuries as technology has advanced. During the Machine Age, it referred to (what we today might call) the "traditional" machining processes, such as turning, boring, drilling, milling, broaching, sawing, shaping, planing, reaming, and tapping, or sometimes to grinding. Since the advent of new technologies such as electrical discharge machining, electrochemical machining, electron beam machining, photochemical machining, and ultrasonic machining, the retronym "conventional machining" can be used to differentiate the classic technologies from the newer ones. The term "machining" without qualification usually implies conventional machining.

Contents [hide] 1 Machining operations 1.1 Circle interpolating 2 Overview of machining technology 2.1 Types of machining operation 2.2 The cutting tool 3 Cutting conditions 4 Stages in metal cutting 5 See also 6 References 7 Further reading 8 External links

[edit] Machining operations

Making a shipboard manhole cover in the machine shop of the aircraft carrier USS John C. Stennis.The three principal machining processes are classified as turning, drilling and milling. Other operations falling into miscellaneous categories include shaping, planing, boring, broaching and sawing.

Turning operations are operations that rotate the workpiece as the primary method of moving metal against the cutting tool. Lathes are the principal machine tool used in turning. Milling operations are operations in which the cutting tool rotates to bring cutting edges to bear against the workpiece. Milling machines are the principal machine tool used in milling. Drilling operations are operations in which holes are produced or refined by bringing a rotating cutter with cutting edges at the lower extremity into contact with the workpiece. Drilling operations are done primarily in drill presses but sometimes on lathes or mills. Miscellaneous operations are operations that strictly speaking may not be machining operations in that they may not be swarf producing operations but these operations are performed at a typical machine tool. Burnishing is an example of a miscellaneous operation. Burnishing produces no swarf but can be performed at a lathe, mill, or drill press. An unfinished workpiece requiring machining will need to have some material cut away to create a finished product. A finished product would be a workpiece that meets the specifications set out for that workpiece by engineering drawings or blueprints. For example, a workpiece may be required to have a specific outside diameter. A lathe is a machine tool that can be used to create that diameter by rotating a metal workpiece, so that a cutting tool can cut metal away, creating a smooth, round surface matching the required diameter and surface finish. A drill can be used to remove metal in the shape of a cylindrical hole. Other tools that may be used for various types of metal removal are milling machines, saws, and grinding tools. Many of these same techniques are used in woodworking.

More recent, advanced machining techniques include electrical discharge machining (EDM), electro-chemical erosion, laser, or water jet cutting to shape metal workpieces.

As a commercial venture, machining is generally performed in a machine shop, which consists of one or more workrooms containing major machine tools. Although a machine shop can be a stand-alone operation, many businesses maintain internal machine shops which support specialized needs of the business.

Machining requires attention to many details for a workpiece to meet the specifications set out in the engineering drawings or blueprints. Beside the obvious problems related to correct dimensions, there is the problem of achieving the correct finish or surface smoothness on the workpiece. The inferior finish found on the machined surface of a workpiece may be caused by incorrect clamping, a dull tool, or inappropriate presentation of a tool. Frequently, this poor surface finish, known as chatter, is evident by an undulating or irregular finish, and the appearance of waves on the machined surfaces of the workpiece.

Basic machining process.[edit] Circle interpolating

The orbital drilling principleCircle interpolating, also known as orbital drilling, is a process for creating holes using machine cutters.

orbital drilling is based on rotating a cutting tool around its own axis and simultaneously about a centre axis which is off-set from the axis of the cutting tool. The cutting tool can then be moved simultaneously in an axial direction to drill or machine a hole – and/or combined with an arbitrary sidewards motion to machine an opening or cavity.

By adjusting the off-set, a cutting tool of a specific diameter can be used to drill holes of different diameters as illustrated. This implies that the cutting tool inventory can be substantially reduced.

The term orbital drilling comes from that the cutting tool “orbits” around the hole center. The mechanically forced, dynamic offset in orbital drilling has several advantages compared to conventional drilling that drastically increases the hole precision. The lower thrust force results in a burr-less hole when drilling in metals. When drilling in composite materials the problem with delamination is eliminated.[1]

[edit] Overview of machining technology Machining is not just one process; it is a group of processes. The common feature is the use of a cutting tool to form a chip that is removed from the workpart, called swarf . To perform the operation, relative motion is required between the tool and work. This relative motion is achieved in most machining operation by means of a primary motion, called "cutting speed" and a secondary motion called "feed'". The shape of the tool and its penetration into the work surface, combined with these motions, produce the desired shape of the resulting work surface.

[edit] Types of machining operation There are many kinds of machining operations, each of which is capable of generating a certain part geometry and surface texture.

In turning, a cutting tool with a single cutting edge is used to remove material from a rotating workpiece to generate a cylindrical shape. The speed motion in turning is provided by the rotating workpart, and the feed motion is achieved by the cutting tool moving slowly in a direction parallel to the axis of rotation of the workpiece.

Drilling is used to create a round hole. It is accomplished by a rotating tool that is typically has two cutting edges. The tool is fed in a direction parallel to its axis of rotation into the workpart to form the round hole.

In boring, the tool is used to enlarge an already available hole. It is a fine finishing operation used in the final stages of product manufacture.

In milling, a rotating tool with multiple cutting edges is moved slowly relative to the material to generate a plane or straight surface. The direction of the feed motion is perpendicular to the tool's axis of rotation. The speed motion is provided by the rotating milling cutter. The two basic forms of milling are —

Peripheral milling Face milling Other conventional machining operations include shaping, planing, broaching and sawing. Also, grinding and similar abrasive operations are often included within the category of machining.

[edit] The cutting tool Main article: Cutting tool (machining)

A "numerical controlled machining cell machinist" monitors a B-1B aircraft part being manufactured.A cutting tool has one or more sharp cutting edges and is made of a material that is harder than the work material. The cutting edge serves to separate chip from the parent work material. Connected to the cutting edge are the two surfaces of the tool —

The rake face; and The flank. The rake face which directs the flow of newly formed chip, is oriented at a certain angle is called the rake angle "α". It is measured relative to the plane perpendicular to the work surface. The rake angle can be positive or negative. The flank of the tool provides a clearance between the tool and the newly formed work surface, thus protecting the surface from abrasion, which would degrade the finish. This angle between the work surface and the flank surface is called the relief angle. There are two basic types of cutting tools —

Single point tool; and Multiple-cutting-edge tool. A single point tool has one cutting edge and is used for turning, boreing and planing. During machining, the point of the tool penetrates below the original work surface of the workpart. The point is sometimes rounded to a certain radius, called the nose radius.

Multiple-cutting-edge tools have more than one cutting edge and usually achieve their motion relative to the workpart by rotating. Drilling and milling uses rotating multiple-cutting-edge tools. Although the shapes of these tools are different from a single-point tool, many elements of tool geometry are similar.

[edit] Cutting conditions Relative motion is required between the tool and work to perform a machining operation. The primary motion is accomplished at a certain cutting speed. In addition, the tool must be moved laterally across the work. This is a much slower motion, called the feed. The remaining dimension of the cut is the penetration of the cutting tool below the original work surface, called the depth of cut. Collectively, speed, feed, and depth of cut are called the cutting conditions. They form the three dimensions of the machining process, and for certain operations, their product can be used to obtain the material removal rate for the process —

where —

— the material removal rate in mm3/s, (in3/s), 
— the cutting speed in m/s, (ft/min), 
— the feed in mm, (in), 
— the depth of cut in mm, (in). 

Note: All units must be converted to the corresponding decimal (or USCU) units. [edit] Stages in metal cutting Machining operations usually divide into two categories, distinguished by purpose and cutting conditions:

Roughing cuts, and Finishing cuts. Roughing cuts are used to remove large amount of material from the starting workpart as rapidly as possible, in order to produce a shape close to the desired form, but leaving some material on the piece for a subsequent finishing operation. Finishing cuts are used to complete the part and achieve the final dimension, tolerances, and surface finish. In production machining jobs, one or more roughing cuts are usually performed on the work, followed by one or two finishing cuts. Roughing operations are done at high feeds and depths — feeds of .04-1.25 mm/rev (0.015-0.050 in/rev) and depths of 2.5-20 mm (0.100-0.750 in) are typical. Finishing operations are carried out at low feeds and depths - feeds of 0.0125-0.04 mm/rev (0.0005-0.0015 in/rev) and depths of 0.75-2.0 mm (0.030-0.075 in) are typical. Cutting speeds are lower in roughing than in finishing.

A cutting fluid is often applied to the machining operation to cool and lubricate the cutting tool. Determining whether a cutting fluid should be used, and, if so, choosing the proper cutting fluid, is usually included within the scope of cutting condition.

Today other forms of metal cutting are becoming increasingly popular. An example of this is water jet cutting. Water jet cutting involves pressurized water in excess of 90,000 PSI and is able to cut metal and have a finished product. This process, is called cold cutting, and it increases efficiency as opposed to laser and plasma cutting.

[edit] See also Abrasive flow machining Design for manufacturability for CNC machining Electrical discharge machining Electrochemical machining Electron beam machining Machinability Machining vibrations Photochemical machining Ultrasonic machining

Vacuum engineering deals with technological processes and equipment that use vacuum to achieve better results than those run under atmospheric pressure. The most widespread applications of vacuum technology are:

Pyrolytic Chromium Carbide Coating Antireflecting glass Glass colouring Vacuum impregnation Vacuum coating Vacuum drying Vacuum coaters are capable of applying various types of coatings on metal, glass, plastic or ceramic surfaces, providing best quality, thickness and color uniformity ever. Vacuum dryers can be used for delicate materials and save up significant energy quantities due to lower drying temperatures.

Leak From Wikipedia, the free encyclopedia Jump to: navigation, search For other uses, see Leak (disambiguation).

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A leak is a hole or other opening, usually unintended and therefore undesired, in a container or fluid-containing system, such as a tank or a ship's hull, through which the contents of the container can escape or outside matter can enter the container. The word "leak" is also used as a verb; matter going through the opening is said to leak. The entry, exit, or exchange of matter through the leak is called leakage, the subject of another disambiguation article. The matter leaking in or out can be gas, liquid, a highly viscous paste, or even a solid such as a powdered or granular solid or other solid particles. A leak can be between two (or more) fluid-containing systems, allowing transfer of matter from either system to the other, or even an exchange of matter between them. Leakage of matter into a container or other system could be called inleakage. Leakage of matter out of a container or other system could be called outleakage. The presence of a leak does not necessarily mean there is always leakage of matter; it just implies there could be leakage through the opening. If the container or system is empty in an empty environment, there is no leakage at that time. Sometimes the word "leak" is used to refer to leakage in a certain situation, for example the passing or making public of secret information.

Contents [hide] 1 Leak types and possible causes 2 Leak testing 3 Corrective action for leaks 4 See also 5 References

[edit] Leak types and possible causes Types of leak openings include a puncture, gash, rust or other corrosion hole, very tiny pinhole leak (possibly in imperfect welds), crack or microcrack, or inadequate sealing between components or parts joined together. When there is a puncture, the size and shape of the leak can often be seen, but in many other cases, the size and shape of the leak opening may not be so obvious. In many cases, the location of a leak can be determined by seeing material drip out at a certain place, although the leak opening itself is not obvious. In some cases, it may known or suspected there is a leak, but even the location of the leak is not known. Since leak openings are often so irregular, leaks are sometimes sized by the leakage rate, as in volume of fluid leaked per time, rather than the size of the opening.

Common types of leaks for many people include leaks in vehicle tires, causing air to leak out resulting in flat tires, and leaks in containers, spilling the contents. Leaks can occur or develop in many different kinds of household, building, vehicle, marine, aircraft, or industrial fluid systems, whether the fluid is a gas or liquid. Leaks in vehicle hydraulic systems such as brake or power steering lines could cause outleakage of brake or power steering fluid resulting in failure of the brakes, power steering, or other hydraulic system. Also possible are leaks of engine coolant - particularly in the radiator and at the water pump seal, transmission fluid, motor oil, and refrigerant in the air conditioning system. Some of these vehicle fluids have different colors to help identify the type of leaking fluid.

The water supply system or a wastewater system in a house or other building may have a leak in any of numerous locations, causing dripping out or spillage of the water. Gas leaks, e.g. in natural gas lines allow flammable and potentially explosive gas to leak out, resulting in a hazardous situation. Leaks of refrigerant may occur in refrigerators or air conditioning systems, large and small. Some industrial plants, especially chemical and power plants, have numerous fluid systems containing many types of liquid or gas chemicals, sometimes at high temperature and/or pressure. An example of a possible industrial location of a leak between two fluid systems includes a leak between the shell and tube sides in a heat exchanger, potentially contaminating either or both fluid systems with the other fluid. A system holding a full or partial vacuum may have a leak causing inleakage of air from the outside. Hazmat procedures and/or teams may become involved when leakage or spillage of hazardous materials occurs. Leaks while transporting hazardous materials could result in danger; for example, when accidents occur. However, even leakage of steam can be dangerous because of the high temperature and energy of the steam.

Leakage of air or other gas out of hot air balloons, dirigibles, or cabins of airplanes could present dangerous situations. A leak could even be inside a body, such as a hole in the septum between heart ventricles causing an exchange of oxygenated and deoxygenated blood, or a fistula between bodily cavities such as between vagina and rectum.

There can be numerous causes of leaks. Leaks can occur from the outset even during construction or initial manufacture/assembly of fluid systems. Pipes, tubing, valves, fittings, or other components may be improperly joined or welded together. Components with threads may be improperly screwed together. Leaks can be caused by damage; for example, punctures or fracture. Often leaks are the result of deterioration of materials from wear or aging, such as rusting or other corrosion or decomposition of elastomers or similar polymer materials used as gaskets or other seals. For example, wearing out of faucet washers causes water to leak at the faucets. Cracks may result from either outright damage, or wearing out by stress such as fatigue failure or corrosion such as stress corrosion cracking. Wearing out of a surface between a disk and its seat in a valve could cause a leak between ports (valve inlets or outlets). Wearing out of packing around a turning valve stem or rotating centrifugal pump shaft could develop into fluid outleakage into the environment. For some frequently operating centrifugal pumps, such leakage is so expected that provisions are made for carrying away the leakage. Similarly, wearing out of seals or packing around piston-driven pumps could also develop into outleakage to the environment.

The pressure difference between both sides of the leak can affect the movement of material through the leak. Fluids will commonly move from the higher pressure side to the lower pressure side. The larger the pressure difference, the more leakage there will typically be. The fluid pressures on both sides include the hydrostatic pressure, which is pressure due to the weight from the height of fluid level above the leak. When the pressures are about equal, there can be an exchange of fluids between both sides, or little to no net movement of fluid across the leak.

[edit] Leak testing Containers, vessels, enclosures, or other fluid system are sometimes tested for leaks - to see if there is any leakage and to find where the leaks are so corrective action can be taken. There are several methods for leak testing, depending on the situation. Sometimes leakage of fluid may make a sound which can be detected. Tires, engine radiators, and maybe some other smaller vessels may be tested by pressurizing them with air and submerging them in water to see where air bubbles come out to indicate a leak. If submerging in water is not possible, then pressurization with air followed by covering the area to be tested with a soap solution is done to see if soap bubbles form, which indicate a leak. Other types of testing for gas leaks may involve testing for the outleaking gases with sensors which can detect that gas, for example - special sensing instruments for detecting natural gas. U.S. federal safety law now requires natural gas companies to conduct testing for gas leaks upstream of their customer's gas meters. Where liquids are used, special color dyes may be added to help see the leakage. Other detectable substances in one of the liquids may be tested, such as saline to find a leak in a sea water system, or detectable substances may even be deliberately added to test for leakage.

Newly constructed, fabricated, or repaired systems or other vessels are sometimes tested to verify satisfactory production or repair. Plumbers often test for leaks after working on a water or other fluid system. A vessel or system is sometimes pressure tested by filling with air and the pressure monitored to see if it drops, indicating a leak. A very commonly used test after new construction or repair is a hydrostatic test, sometimes called a pressure test. In a hydrostatic test, a system is pressurized with water to look for a drop in pressure or to see where it leaks out. Helium testing may be done to detect for any very small leakage such as when testing certain diaphragm or bellows valves, which are made to be practically leak-proof. Helium and Hydrogen have very small molecules which can go through very small leaks.

Leak testing is part of the non-destructive test NDT portfolio that can be applied to an art to verify its conformity; depending on material, pressure, leak tightness specifications, different methods can be applied. International standards has been defined to assist in these choices. For example BS EN 1779:1999; it applies to assessment of leak tightness by indication or measurement of gas leakage, but excludes hydrostatic, ultrasonic or electromagnetic methods. Other standards also apply:

BS EN 13184:2001 Non-destructive testing. Leak testing. Pressure change method BS EN 13185:2001 Non-destructive testing. Leak testing. Tracer gas method BS EN 13192:2002 Non-destructive testing. Leak testing. Calibration of reference leaks for gases In shell and tube heat exchangers, Eddy current testing is sometimes done in the tubes to find locations on tubes where there may be leaks or damage which may eventually develop into a leak.

[edit] Corrective action for leaks In complex plants with multiple fluid systems, many interconnecting units holding fluids have isolation valves between them. If there is a leak in a unit, its isolation valves can be shut to "isolate" the unit from the rest of the plant.

Leaks are often repaired by plugging the leaking holes or using a patch to cover them. Leaking tires are often fixed this way. Leaking gaskets, seals, washers, or packing can be replaced. Use of welding, soldering, sealing, or gluing may be other ways to fix leaks. Sometimes, the most practical solution is to replace the leaking unit. Leaking water heaters are often replaced by home or building owners.

If there is a leak in one of the tubes of a shell and tube heat exchanger, that tube can be plugged at both ends with specially sized plugs to isolate the leak. This is done in the plenum(s) at the points where the tube ends connect to the tubesheet(s). Sometimes a damaged but not yet leaking tube is pre-emptively plugged to prevent future leakage. The heat transfer capacity of that tube is lost, but there are usually plenty of other tubes to pick up the heat transfer load.

Powder coating From Wikipedia, the free encyclopedia Jump to: navigation, search

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Powder coating is a type of coating that is applied as a free-flowing, dry powder. The main difference between a conventional liquid paint and a powder coating is that the powder coating does not require a solvent to keep the binder and filler parts in a liquid suspension form. The coating is typically applied electrostatically and is then cured under heat to allow it to flow and form a "skin." The powder may be a thermoplastic or a thermoset polymer. It is usually used to create a hard finish that is tougher than conventional paint. Powder coating is mainly used for coating of metals, such as "whiteware", aluminium extrusions, and automobile and bicycle parts. Newer technologies allow other materials, such as MDF (medium-density fibreboard), to be powder coated using different methods.

Contents [hide] 1 Advantages and disadvantages of powder coating 2 Types of powder coatings 3 The powder coating process 4 Removing Powder Coating 5 See also 6 External links

[edit] Advantages and disadvantages of powder coating There are several advantages of powder coating over conventional liquid coatings:

Powder coatings emit zero or near zero volatile organic compounds (VOC). Powder coatings can produce much thicker coatings than conventional liquid coatings without running or sagging. Powder coating overspray can be recycled and thus it is possible to achieve nearly 100% use of the coating. Powder coating production lines produce less hazardous waste than conventional liquid coatings. Capital equipment and operating costs for a powder line are generally less than for conventional liquid lines. Powder coated items generally have fewer appearance differences between horizontally coated surfaces and vertically coated surfaces than liquid coated items. A wide range of specialty effects is easily accomplished which would be impossible to achieve with other coating processes. While powder coatings have many advantages over other coating processes, there are limitations to the technology. While it is relatively easy to apply thick coatings which have smooth, texture-free surfaces, it is not as easy to apply smooth thin films. As the film thickness is reduced, the film becomes more and more orange peeled in texture due to the particle size and TG (glass transition temperature) of the powder. Also powder coatings will break down when exposed to uv rays between 5 to 10 years.

For optimum material handling and ease of application, most powder coatings have a particle size in the range of 30 to 50 μm and a TG > 40°C. For such powder coatings, film build-ups of greater than 50 μm may be required to obtain an acceptably smooth film. The surface texture which is considered desirable or acceptable depends on the end product. Many manufacturers actually prefer to have a certain degree of orange peel since it helps to hide metal defects that have occurred during manufacture, and the resulting coating is less prone to show fingerprints.

There are very specialized operations where powder coatings of less than 30 micrometres or with a TG < 40°C are used in order to produce smooth thin films. One variation of the dry powder coating process, the Powder Slurry process, combines the advantages of powder coatings and liquid coatings by dispersing very fine powders of 1–5 micrometre particle size into water, which then allows very smooth, low film thickness coatings to be produced.

Powder coatings have a major advantage in that the overspray can be recycled. However, if multiple colors are being sprayed in a single spray booth, this may limit the ability to recycle the overspray.

[edit] Types of powder coatings There are two main categories of powder coatings: Thermosets and thermoplastics. The thermosetting variety incorporates a cross-linker into the formulation. When the powder is baked, it reacts with other chemical groups in the powder polymer and increases the molecular weight and improves the performance properties. The thermoplastic variety does not undergo any additional reactions during the baking process, but rather only flows out into the final coating.

The most common polymers used are polyester, polyester-epoxy (known as hybrid), straight epoxy (Fusion bonded epoxy) and acrylics.

Production:

The polymer granules are mixed with hardener, pigments and other powder ingredients in a mixer The mixture is heated in an extruder The extruded mixture is rolled flat, cooled and broken into small chips The chips are milled to make a fine powder [edit] The powder coating process The powder coating process involves three basic steps:

Part preparation or the Pre treatment The powder application Curing Part Preparation Processes & Equipment Removal of oil, soil, lubrication greases, metal oxides, welding scales etc. is essential prior to the powder coating process. It can be done by a variety of chemical and mechanical methods. The selection of the method depends on the size and the material of the part to be powder coated, the type of soil to be removed and the performance requirement of the finished product.

Chemical pre-treatments involve the use of phosphates or chromates in submersion or spray application. These often occur in multiple stages and consist of degreasing, etching, de-smutting, various rinses and the final phosphating or chromating of the substrate. The pre-treatment process both cleans and improves bonding of the powder to the metal. Recent additional processes have been developed that avoid the use of chromates, as these can be toxic to the environment. Titanium Zirconium and Silanes offer similar performance against corrosion and adhesion of the powder.

Another method of preparing the surface prior to coating is known as abrasive blasting or Sandblasting and shot blasting. Blast media and blasting abrasives are used to provide surface texturing and preparation, etching, finishing, and degreasing for products made of wood, plastic, or glass. The most important properties to consider are chemical composition and density; particle shape and size; and impact resistance.

Silicon carbide grit blast media is brittle, sharp, and suitable for grinding metals and low-tensile strength, non-metallic materials. Plastic media blast equipment uses plastic abrasives that are sensitive to substrates such as aluminum, but still suitable for de-coating and surface finishing. Sand blast media uses high-purity crystals that have low-metal content. Glass bead blast media contains glass beads of various sizes.

Cast steel shot or steel grit is used to clean and prepare the surface before coating. Shot blasting recycles the media and is environmentally friendly. This method of preparation is highly efficient on steel parts such as I-beams, angles, pipes, tubes and large fabricated pieces.

Different powder coating applications can require alternative methods of preparation such as abrasive blasting prior to coating. The online consumer market typically offers media blasting services coupled with their coating services at additional costs.

Powder Application Processes The most common way of applying the powder coating to metal objects is to spray the powder using an electrostatic gun, or Corona gun. The gun imparts a positive electric charge on the powder, which is then sprayed towards the grounded object by mechanical or compressed air spraying and then accelerated toward the workpiece by the powerful electrostatic charge. There are a wide variety of spray nozzles available for use in electrostatic coating. the type of nozzle used will depend on the shape of the workpiece to be painted and the consistency of the paint. The object is then heated, and the powder melts into a uniform film, and is then cooled to form a hard coating. It is also common to heat the metal first and spray the powder onto the hot substrate. Preheating can help to achieve a more uniform finish but can also create other problems, such as runs caused by excess powder. See the article "Fusion Bonded Epoxy Coatings"

Another type of gun is called a Tribo gun, which charges the powder by (triboelectric) friction. In this case, the powder picks up a positive charge while rubbing along the wall of a Teflon tube inside the barrel of the gun. These charged powder particles then adhere to the grounded substrate. Using a Tribo gun requires a different formulation of powder than the more common Corona guns. Tribo guns are not subject to some of the problems associated with Corona guns, however, such as back ionization and the Faraday Cage Effect.

Powder can also be applied using specifically adapted electrostatic discs.

Another method of applying powder coating, called the Fluidized Bed method, is by heating the substrate and then dipping it into an aerated, powder-filled bed. The powder sticks and melts to the hot object. Further heating is usually required to finish curing the coating. This method is generally used when the desired thickness of coating is to exceed 300 micrometres. This is how most dishwasher racks are coated.

Electrostatic Fluidized Bed Coating: Electrostatic fluidized bed application uses the same fluidizing technique and the conventional fluidized bed dip process but with much less powder depth in the bed. An electrostatic charging media is placed inside the bed so that the powder material becomes charged as the fluidizing air lifts it up. Charged particles of powder move upward and form a cloud of charged powder above the fluid bed. When a grounded part is passed through the charged cloud the particles will be attracted to its surface. The parts are not preheated as they are for the conventional fluidized bed dip process.

Electrostatic magnetic Brush (EMB) Coating: an innovative coating method for flat materials that applies powder coating with roller technique, enabling relative high speeds and a very accurate layer thickness between 5 and 100 micrometre. The base for this process is conventional copier technology . Currently in use in some high- tech coating applications and very promising for commercial powder coating on flat substrates ( steel, Aluminium, MDF, paper, board) as well in sheet to sheet and/or roll to roll processes. This process can potentially be integrated in any existing coating line.

Curing

When a thermoset powder is exposed to elevated temperature, it begins to melt, flows out, and then chemically reacts to form a higher molecular weight polymer in a network-like structure. This cure process, called crosslinking, requires a certain degree of temperature for a certain length of time in order to reach full cure and establish the full film properties for which the material was designed. Normally the powders cure at 200°C (390°F) in 10 minutes. The curing schedule could vary according to the manufacturer's specifications.

The application of energy to the product to be cured can be accomplished by convection cure ovens or infrared cure ovens.

[edit] Removing Powder Coating Methylene Chloride is generally effective at removing powder coating, however most other organic solvents (Acetone, thinners, etc.) are completely ineffective. Most recently the suspected human carcinogen Methylene Chloride is being replaced by Benzyl alcohol with great success. Powder coating can also be removed with abrasive blasting. 98% sulfuric acid commercial grade also removes powder coating film.[citation needed] Certain low grade powder coats can be removed with steel wool, though this might be a more labor- intensive process than desired.